I'm cooling one 600 with a 4" fan and ducts, and it's just not quite enough. Switching to 6" soon, along with re-routing ducts so there are less bends (a big factor). Also I find that the only way to get a 600 nice and close to the plants is to have dedicated light ventilation. And you need it if you are running CO2. If I could only give one bit of advice it would be to not skimp on ventilation. For HID, you really need to spend as much on venting as lighting.

Nice write-up rhizome.